1. Field of the Invention
The present invention relates to a method of removing mercury, and more specifically, to a method of efficiently removing mercury in a simple manner from a liquid hydrocarbon containing mercury.
2. Description of the Prior Art
Natural gas liquid (NGL), also referred to as natural gas condensate, obtained by removing liquefied petroleum gas from a natural gas-field product generally contains mercury in about 2 to several thousands ppb, although depending on the production areas. Therefore, liquid hydrocarbons obtained by distilling NGL often contain mercury.
When a liquid hydrocarbon containing mercury is used as row chemical materials, mercury therein deteriorates the catalytic activity of a hydrogenation catalyst or corrodes materials of apparatus because mercury forms amalgams with the catalyst components and materials of apparatus such as palladium, platinum, copper, aluminum, etc. In addition, the presence of dissolved oxygen absorbed due to NGL-air contact temporarily decreases the mercury concentration of NGL by changing the dissolved mercury to insoluble mercury in NGL. However, the insoluble mercury becomes soluble in NGL with the passage of time to increase the mercury concentration of NGL again.
Therefore, it has been keenly desired to develop a method of efficiently removing mercury from liquid hydrocarbons while preventing the mercury concentration from increasing again.
Japanese Patent Publication No. 7-91544 discloses to remove mercury using an adsorbent comprising a copper halide supported on a carrier such as activated clay. Although the mercury concentration of the treated liquid hydrocarbon is reduced to 4 to 6 ppb in some cases, only reduced to about 60 to 80 ppb in most cases. In addition, the production of the adsorbent requires many production steps such as carrying, drying and sintering steps. Further, the adsorbent is susceptible to change in its properties due to several factors to adversely affect the removal efficiency of mercury from the liquid hydrocarbon. Thus, an adsorbent having a stable mercury adsorptivity has been difficult to prepare.
Japanese Patent Application Laid-Open No. 4-331287 proposes to extract mercury by an extractant comprising water dissolving an polyhydric alcohol in an amount 3 volume % or more based on water. Although the mercury concentration of the treated liquid hydrocarbon may be 10 ppb or less in some cases, usually in insufficient level of about 10 to 30 ppb. Further, the method is extremely energy-consuming because the recovery of the polyhydric alcohol from the liquid hydrocarbon and aqueous solution and the purification of recovered polyhydric alcohol are required.
Japanese Patent Publication No. 4-28040 discloses a removing method including a step of treating a liquid hydrocarbon containing mercury with a sulfur compound represented by the formula: MM'S, wherein M and M' are identical or different and are each hydrogen, alkali metal or ammonium group, and a step of contacting the liquid hydrocarbon with an adsorbent containing at least one heavy metal sulfide. However, this method is rather complicated, because mercury is removed by blowing hydrogen sulfide gas into the liquid hydrocarbon and subsequently adsorbing mercury onto a adsorbent comprising sulfide of molybdenum and cobalt supported on alumina. Also, the adsorption step requires troublesome preparation of the adsorbent and strict control of operating conditions. Further, as noted above, the adsorbent is susceptible to change in its properties due to several factors to adversely affect the removal efficiency of mercury from the liquid hydrocarbon. Thus, the proposed method involves a difficulty of preparing an adsorbent having a stable mercury adsorptivity.
Japanese Patent Publication No. 6-89338 teaches to treat at 40.degree. C. or higher a liquid hydrocarbon containing mercury with an aqueous solution of a sulfur compound represented by the formula: MM'Sx, wherein M is alkali metal or ammonium group, M' is hydrogen, alkali metal or ammonium group, and x is a number of 1 to 6. However, the removal of mercury is insufficient, because the mercury concentration after treatment is as high as 30 to 170 ppb when treated at 40.degree. C. or less. Even when treated at 100 to 120.degree. C., the residual mercury concentration is as high as 30 ppb or more, although about 30 ppb in some cases.
In addition, in the methods of Japanese Patent Publication Nos. 4-28040 and 6-89338, the mercury concentration temporarily reduced often comes to increase again.